1. Field of the Invention:
The present invention relates to a polarity switching control method for improving stability during polarity switching in consumable electrode AC pulse arc welding.
2. Description of the Related Art:
For consumable electrode AC pulse arc welding (referred to as “AC pulse arc welding” below), a welding wire is supplied at constant speed, and two periods, i.e. an electrode-positive polarity period Tep and an electrode-negative polarity period Ten, are caused to alternate with each other. During the electrode-positive polarity period Tep, peak current Ip and base current Ib are applied, whereas during the electrode-negative polarity period Ten, electrode-negative current In is applied. In order to achieve high quality welding, it is important in AC pulse arc welding that polarity switching is performed properly so that no arc interruption occurs. To this end, conventionally, a high voltage is applied between the welding wire and the base metal in switching the polarity, as described below.
FIG. 4 is a timing chart illustrating conventional AC pulse arc welding. Specifically, Graph 4(A) shows the waveform of a welding current Iw, Graph 4(B) the waveform of a welding voltage Vw, and Graph 4(C) the waveform of a polarity switching signal Cp. When the polarity switching signal Cp changes from High level to Low level, a switching operation from the electrode-positive polarity period Tep to the electrode-negative polarity period Ten is performed. When the signal changes from Low level to High level, a switching operation from the electrode-negative polarity period Ten to the electrode-positive polarity period Tep is performed. A more specific explanation is given below.
(1) Period from Time Point t1 to Time Point t2.
As shown in Graph 4(C), the polarity switching signal Cp assumes Low level at Time Point t1, whereupon the base current Ib of the electrode-positive polarity period Tep decreases, as shown in Graph 4(A), toward a predetermined polarity switching current value Ic. At Time Point t2, the welding current Iw becomes equal to the polarity switching current value Ic, whereupon a high voltage is applied, and as shown in Graph 4(B), the polarity is switched to the electrode-negative polarity period Ten.
(2) Period from Time Point t2 to Time Point t3.
This period is an electrode-negative polarity EN. As shown in Graph 4(A), a predetermined electrode-negative current In is supplied, and an electrode-negative voltage Vn, as shown in Graph 4(B), is applied.
(3) Period from Time Point t3 to Time Point t4.
At Time Point t3, as shown in Graph 4(C), the polarity switching signal Cp changes to assume High level, upon which the electrode-negative current In decreases toward the polarity switching current value Ic as shown in Graph 4(A). At Time Point t4, the welding current Iw becomes equal to the polarity switching current value Ic as shown in Graph 4(A), upon which a high voltage is applied and the polarity is switched to the electrode-positive polarity EP as shown in Graph 4(B).
(4) Period from Time Point t4 to Time Point t5.
This period is a peak period Tp, in which the electrode-positive polarity EP is adopted. As shown in Graph 4(A) a predetermined peak current IP is supplied, and a peak voltage Vp, as shown in Graph 4(B), is applied.
(5) Period from Time Point t5 through Time Point t8.
This period is a base period Tb determined by feed-back control. During this period, the electrode-positive polarity EP is adopted. As shown in Graph 4(A), a predetermined base current Ib is supplied, and a base voltage Vb, as shown in Graph 4(B), is applied.
(6) Period from Time Point t8 to Time Point t9.
In this period, the process goes back to the (1) operation described above.
(7) Period from Time Point t6 to Time Point t7.
This period is included in the above-mentioned base period Tb. When short-circuiting occurs between the welding wire and the base metal, the welding voltage Vw assumes a low short-circuit voltage value, as shown in Graph 4(B). At the same time, as shown in Graph 4(A), an increasing short-circuit current Is is applied to terminate the short-circuiting quickly and to generate the arc again. As a result, the arc recurs at Time Point t7. It should be noted that short-circuiting occurs not only in the base period Tb but also in the peak period Tp as well as in the electrode-negative polarity period Ten. For these occasions, the same operation of applying the short-circuit current Is is supplied.
As seen from FIG. 4, the period from Time Point t2 through Time Point t4 is an electrode-negative polarity period Ten, and the period from Time Point t4 through Time Point t9 is an electrode-positive polarity period Tep. As described, the welding current Iw is decreased to the polarity switching current value Ic at the time of polarity switching. This is a conventionally common practice for protecting a polarity switching device from a surge voltage to be generated by polarity switching operation. For more information, see JP-A-H58-38664 and JP-A-2005-349406, for example.
In the above conventional AC pulse arc welding, stable polarity switching is achieved by applying a high voltage at the time of polarity switching, thereby preventing arc interruption. However, if short-circuiting occurs during the polarity switching, there is a high likelihood for occurrence of prolonged short-circuiting and arc interruption. This problem is descried below with reference to FIG. 5.
FIG. 5 shows how waveforms change when short-circuiting occurs during polarity switching. The chart corresponds to FIG. 4 described above. In FIG. 5, processes from Time Point t1 through Time Point t6 are identical with these in FIG. 4 and so will not be described again. The process after Time Point t6 will be described.
When short-circuiting occurs at Time Point t6, the welding voltage Vw assumes a low short-circuit voltage value as shown in FIG. 5(B) whereas the welding current Iw assumes the short-circuit current Is, i.e. increases as shown in FIG. 5(A). At Time Point t7, the polarity switching signal Cp changes to assume Low level as shown in FIG. 5(C), the operation begins to shift to the electrode-negative polarity EN, and therefore, the welding current Iw (short-circuit current Is) decreases as shown in FIG. 5(A) toward the polarity switching current value Ic. However, as shown in FIG. 5(B), because of the short circuit situation, the decrease in the welding current Iw is substantially slower as shown in FIG. 5(A) than the decrease under an arcing situation. Since the welding current Iw is decreased in spite of the short-circuiting situation, the short-circuiting becomes less likely to be terminated during the period from Time Point t7 to Time Point t8.
At Time Point t8, the welding current Iw becomes equal to the polarity switching current value Ic as shown in FIG. 5(A), whereupon the operational condition switches to electrode-negative polarity EN. However, as shown in FIG. 5(B), the short-circuiting situation continues even after the polarity changes. Thus, as shown in FIG. 5(A), the short-circuit current Is which has a large current value continues to be drawn at electrode-negative polarity EN even after Time Point t8, until Time Point t9, where the wire tip is melted and the arc ceases.
When short-circuiting occurs during polarity switching, the decreasing of the welding current Iw causes the short-circuiting to persist, thereby making the arc unstable or even broken by melting. Behind such a phenomenon is a fact that the pulse arc welding is not a short-circuit transfer welding. In pulse arc welding, it is necessary to quickly terminate short-circuiting and regenerate the arc for stable arcing. In short-circuit transfer welding such as CO2 welding or MAG welding, short-circuiting occurs periodically about 100 times per second, so polarity switching can be performed successfully during the short-circuiting period. In pulse arc welding, however, short-circuiting occurs irregularly. In light of this, the polarity switching is to be performed while the arc is being generated.